I would also be interested if/when an export plugin is available.
I think the closest you can do is to install the OAI-PMH repository plugin, and then use the OAI-PMH repository to get XML of the metadata. To get XML records, you would put in your base URL plus /oai-pmh-repository/request?verb=ListRecords&metadataPrefix=oai_dc . There is a maximum number of records that will be returned. To change that, change the maximum number of records to display per page which will also increase your memory usage. It's clunky.
I know this has come up before, but all I know about with previous discussions on exporting are
http://omeka.org/forums/topic/export-dublin-core-from-omeka and
https://groups.google.com/forum/#!topic/omeka-dev/IhV9Hlyw1rE .
Am I missing the obvious, or is anyone working on an export plugin?
-Wilhelmina Randtke